Tens of thousands of British nationals in four EU member states have yet to apply for post-Brexit residence, meaning they risk losing the right to live and work there unless they file their demands within 30 days.

UK citizens living in France, Malta, Luxembourg and Latvia have until 30 June to apply to secure their post-Brexit rights. The Netherlands did have the same deadline, but on Monday extended it to 1 October.

Under the terms of the withdrawal agreement, UK citizens who were legally resident in one of the EU’s 27 member states at the end of the transition period on 31 December last year are eligible for permanent residence, protecting their basic rights.

Fourteen countries, including Spain, Germany, Portugal and Italy, opted for systems that automatically confer a new post-Brexit residence status on legally resident Britons, with no risk of losing rights if any administrative deadline is missed.

The other 13, however, operate systems under which UK nationals must formally apply for their new residence status, including the four who set an earlier cutoff date of 30 June.

“That’s only a month to go before a hard deadline, after which a lot of people could lose their rights,” said Michaela Benson, a professor of public sociology at Lancaster University, who has specialised in studying British residents in the EU.

“We urgently need more communication – from the UK, the EU and member states – to get in touch, especially with hard-to-reach, vulnerable UK citizens who risk missing a vital cutoff point.”

Benson said the people at risk of “falling through the gaps” were often the most vulnerable. “Those who have stayed off the radar for whatever reason – maybe because they couldn’t prove they were lawfully resident when they had to,” she said.

“The ones to worry about are those who are just scraping by, perhaps in remote areas. They are not likely to come forward of their own accord. There will also be homeless British people, sick British people, British children in care.”

According to the EU/UK joint committee on citizens’ rights, whose latest report, dated 28 April, was released on Friday, an estimated 298,000 Britons live in the 13 countries with so-called constitutive systems. Only 190,000 have applied for their new status.

In the four countries with a June deadline, 25,500 of France’s estimated 148,000 British residents have yet to register, nearly 800 Britons still have to apply in Latvia, 1,700 in Luxembourg and 5,300 in Malta.

In the Netherlands, 3,000 have yet to apply before the new 1 October date, along with 12,000 in Finland and Sweden, which have 30 September deadlines. The cutoff date in Austria, Belgium, Denmark, Hungary, Romania and Slovenia is 31 December.

Benson said the figures were approximate since most were based on data from 2018 and some EU states, such as France, had never required EU residents to register with authorities in order to qualify for public services, so may have significantly underestimated.

“That means it may well have many more British residents than it imagines, just as the UK had many more EU residents than it thought,” she said. “It also makes it much harder to reach them than in places like the Netherlands with up-to-date registers.”

According to the EU’s post-Brexit guidance, “failure to apply in time … may lead to a loss of any entitlement under the withdrawal agreement” in countries with constitutive systems, including the right to continue living in them.

According to the committee’s report, an estimated 762,000 UK citizens live in EU countries with automatic – or “declaratory” – systems for post-Brexit residence status, and nearly 165,000 have registered so far.

Several of these 14 countries also have deadlines for UK residents to register for their new status, but those who do not face a fine rather than loss of rights.